The compound mentioned belongs to the class of substituted phenylacetic acid derivatives which are useful intermediate products for insecticides, pharmaceuticals, and agricultural chemicals. In accordance with their use, emphasis is placed on a high purity of these compounds, because impurities severaly impair the quality and activity of the end products.
Various processes are known for the preparation of substituted phenylacetic acid derivatives, and in particular 2-(4-chlorophenyl)-3-methylbutyric acid.
Thus, according to British Patent 1,528,043, 4-halogenobenzyl halides 4-X-C.sub.6 H.sub.4 -CH.sub.2 X (wherein X is individually Cl, Br, or I) are reacted with approximately an equimolar amount of an alkali metal cyanide and a quaternary ammonium compound. A benzylnitrile derivative is formed, which is reacted with a molar excess of a secondary alkyl chloride in the presence of an alkali metal hydroxide; the product can then be hydrolyzed to the carboxylic acid.
According to C.A. 105 (1986), 78660w, 4-chlorophenyl isopropyl ketone is methylated with a sulfonium salt, the epoxide formed is isomerized with a Lewis or Bronsted acid, and the resulting aldehyde is oxidized to 2-(4-chlorophenyl)-3-methylbutyric acid.
Finally, the preparation of 2-(4-chlorophenyl)-3-methylbutyric acid from 4-chlorophenyl 2-methylpropyl ketone is described in the published Japanese Patent Application 79/84,543. The ketone is converted into an enamine by reaction with a secondary amine and a dehydrating agent.
Reaction of the enamine with the azide of a phosphoric acid ester gives an amidine, hydrolysis of which leads to 2-(4-chlorophenyl)-3-methylbutyric acid.
However, the known processes do not meet all the requirements imposed on a modern chemical process used commercially. In addition to the above-mentioned high purity of the reaction product, it is expected that the starting substances are readily available, no by-products which pollute the environment are formed, and at least good yields are achieved.